A known hydraulic pressure value controller for an automatic transmission directly controls a hydraulic pressure value from a hydraulic pressure source by an electromagnetic valve and the hydraulic pressure value provided to friction engaging elements (friction clutch and friction brake), then each friction engaging element becomes in engaged or disengaged condition.
Specifically, such clutch is controlled to move rapidly within an allowance range from the point where the clutch stars to be moved to the point where the clutch becomes in engaging condition, and further controlled to move slowly by shifting the connection speed once the clutch becomes in engaging condition. As shown in FIG. 11, in the allowance range of a first half of a piston stroke of a clutch piston (hereinbelow referred to as a piston), the friction engaging element is quickly filled with fluid (pre-charge) for increasing a piston speed. After a predetermined pre-charge time, the piston speed is decelerated to almost “zero” on the verge of that the friction engaging element becomes in engaging condition. Then, the piston speed needs to be maintained at a low speed corresponding to the low hydraulic pressure value (stand-by hydraulic pressure value) for keeping the piston at stand-by position. In this way, the automatic transmission needs to be more improved in terms of responsiveness and followability. Thus, the operation speed of transmission may be improved, at the same time, a shift shock may be prevented.
The shift shock will be occurred when the pre-charge pressure or pre-charge time is excess, and the responsiveness and followability will be poor when the pre-charge pressure or pre-charge time is short, so that the pre-charge pressure or pre-charge time need to be set preferably. If an appropriate acceleration time of the piston is set by the setting the pre-charge time preferably in response to the predetermined pre-charge pressure, especially at initial factory setting, individuality on each vehicle due to the environment or fluctuations of the automatic transmission, engine or electromagnetic valve will be reduced, and product quality will be secured.
To realize such pre-charge time control, a method to correctly detect a time when the piston engages with the clutch needs to be presented. As shown in FIG. 11, in normal transmission except a shift change when the vehicle start traveling (N→D, N→R), a rotation number of the input shaft is not changed until an inertia phase at which a rotation change arises inside the automatic transmission, so that an output shaft torque may be detected by a torque sensor in stead of the rotation number of the input shaft, but the fact is that such torque sensor is not used in terms of poor accuracy thereof and the fluctuation of the output shaft torque.
Meanwhile, methods for detecting inflection point of the input shaft rotation are disclosed in Japanese Patent Laid-Open Publications Tokukaihei 6-11026, Tokukaihei 11-351365 and Tokukaihei 9-287657. In Tokukaihei 6-11026, a means for determining an engaging starting point of a friction element used when the vehicle starts moving depending on a rotation number of a turbine is disclosed. In addition, a means for determining a transmission starting point when the rotation number being smaller than the maximum rotation number is detected twice in series is disclosed in Tokukaihei 11-351365. Furthermore, in Tokukaihei 9-287657, a means being combination of aforementioned two known means for determining a starting point of the inertia phase based on a predetermined percentage change of the rotation number obtained from a predetermined time and a predetermined rotation is disclosed.
However, if aforementioned three known means are used to detect the engaging point where the piston engages with the clutch, problems may be occurred that it becomes difficult to prevent the individual difference on each product because various thresholds need to be set based on temperature and each friction engaging element. For example, the preferable pre-charge time in response to the each of friction engaging elements, temperature and the pre-charge pressure may not be set on initial setting before shipment, so that the piston is engaged with the clutch really fast. In this case, the input shaft rotation changes quickly, furthermore, such change may differ depending on each friction engaging element, so that the various thresholds need to be set based on temperature and each friction engaging element.
FIG. 12 illustrates waveforms of each value related to the automatic transmission when the friction engagement element becomes engaged by the movement of the piston at the predetermined pre-charge pressure. It is clear from the actuation structure of the automatic transmission, the piston is pushed and moved by the hydraulic pressure to be engaged with the friction engaging element, then the piston strokes ends, and the torque is transmitted along with that the hydraulic pressure rapidly rises. It is also clear from waveforms in FIG. 12 that a slope of the declining line indicating the rotation number of the turbine, for example, is determined by a factor due to a structure of the friction engaging element. However, it is not sure that a transmitting starting point is obtained as a unique value by the aforementioned known means. Thus, the aforementioned known means may be preferable for determining the inflection point of the individual input shaft, however, there is a limit to practically use such known means for determining the point at which the piston engages with the clutch.
Furthermore, the inflection point of the hydraulic pressure as shown in FIG. 12 may be used as the point at which the piston is engaged with the clutch, in other words, a torque generating point, however, may not correspond to the inflection point of the hydraulic pressure due to various conditions, such as temperature, and various kinds of the friction engaging element.
In consideration of aforementioned problems, the present invention seeks to provide a pre-charge time setting means being high accuracy and applicable regardless of various conditions, such as individual difference between each product, temperature change or various characteristics of the friction engaging element, and a automatic transmission including such pre-charge time setting means.